Portable terminal devices for mobile communications generally use a rod-shaped antenna, which is of a structure convenient for portability, such that it is folded in a device housing in a state of on-hook. Many antennas for satellite communications are in the form of a rod-shaped four-wire helical antenna. (For example, see Japanese laid-open patent publication No. 10-135733.)
A four-wire helical antenna shown in FIG. 1 of the accompanying drawings has radiation elements comprising four helical conductors 71 wound around dielectric cylinder 72 and feeder circuit 73 connected to the lower ends of the radiation elements for supplying high-frequency electric energies each having phase shifted 90 degree from neighboring ones and having the same amplitude. The four-wire helical antenna can transmit and receive a circularly polarized radio wave. Generally, the four-wire helical antenna has a cross-sectional shape as shown in FIG. 2 of the accompanying drawings. Cylinder 72 with the helical conductors wound thereon is covered with radome 81 made of a dielectric material and serving as a protective cover. Radome 81 is fixed to lower cover 82. Cover 82 houses therein feeder circuit 73 connected to the helical conductors. Feeder circuit 73 has a terminal on its lower portion which is connected via rotary joint 8 to a radio unit of a terminal device. The antenna is held upwardly when in use, and downwardly when not in use.
The four-wire helical antenna is required to have a directivity pattern for radiating a radio wave uniformly to the direction above semispherical surface of the device for use in satellite communications. FIG. 3 of the accompanying drawings shows, by way of example, a radiation pattern within an elevation angle of the four-wire helical antenna. The radiation pattern is substantially rotationally symmetric with respect to a bearing direction, and is such a pattern as to give a substantially uniform gain on the upper surface of the semispherical body.
Helical antennas also include an N-wire helical antenna having N helical conductors, other than the four-wire helical antenna. Therefore, other rod-shaped antennas can be used insofar as they can form a radiation pattern similar to the radiation pattern shown in FIG. 3, regardless of the antenna types and kinds.
The rod-shaped antenna for satellite communications which has the radiation pattern shown in FIG. 3 can be held upwardly when in use, i.e., when the terminal device is used in communications, and can be folded downwardly when not in use, i.e., when the terminal device is in the state of on-hook, waiting for calls. When the terminal device is waiting for calls with the rod-shaped antenna being folded downwardly, the sensitivity of the rod-shaped antenna with respect to radio waves traveling through the air is greatly reduced, making it difficult for the terminal device to receive a radio wave indicative of a call from the satellite. Therefore, it is necessary to hold the antenna upwardly while the terminal device is in motion when the terminal device is waiting for calls.